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The liquid circulation velocity in an airlift reactor can be represented by either the linear liquid velocity or by the superficial liquid velocity (Section 2.3.1.2). Like the gas holdup, the liquid circulation rate can also be separated into the overall, riser and downcomer liquid velocities. The average riser and downcomer superficial liquid velocities were measured in this work in the riser and downcomer sections of the airlift, respectively (Section 4.6). With all the liquids, the liquid velocity measurements were made only after a stable value of gas holdup was achieved.

Figure 5.7 shows the effect of superficial gas velocity on the riser and downcomer superficial liquid velocities in the internal loop airlift (1), in the internal loop airlift (2), and in the external loop airlift contactors with water. The given liquid velocity values are an average from three separate runs. Both the riser and downcomer liquid velocities, like the gas holdups, increased with increasing gas flow rate. It was also observed that there was an increase in the absolute difference between the riser and downcomer liquid velocities with increasing gas velocity in all three airlifts. This was similar to the increase in the difference between riser and dovmcomer gas holdups with increasing gas velocity (Figs 5.4 - 5.6). This phenomenon was also observed with the other 19 liquids in the three airlift devices and might be due to a change in the two phase flow regime.

The effect of superficial gas velocity on the riser and downcomer superficial liquid velocities with 82.0% glycerol solution and 0.3% CMC solution are presented in Figures 5.8 and 5.9, respectively. A reduction in the rate of increase of riser and downcomer liquid velocities with increasing gas velocities roughly above 12.8 mm/s was noticed. This is likely to be due to the change in the flow regime from coalesced bubble to turbulent flow with viscous liquids. Merchuk and Stein (1981) explained that in the turbulent flow regime the formation and disengagement of large bubbles lead to greater dissipation of energy, and this would therefore reduce the amount of energy available for liquid circulation. Pollard (1995) also made similar observations in a draft-tube internal loop airlift reactor with baker’s yeast broth (a low viscosity fermentation liquid). But, he observed a decrease in the rate of increase of liquid velocity with increasing gas velocities above about 0.054 m/s. That is a change in the flow regime from coalesced bubble to turbulent flow took place at a higher gas velocity than was in this study. This was due to the low viscosity of the fermentation medium (viscosity -0.001 Pa s).

In all three airlift devices, the dow ncom er liquid velocity was higher than the riser liquid velocity (Figs 5.7 - 5.9). This was expected since the dow ncom er to riser cross sectional area (Ad/Ar) ratios, for all three airlifts, were below unity. A sm aller dow ncom er cross sectional area (com pared to the riser area) gives a higher dow ncom er liquid velocity. In the external loop airlift device the dow ncom er liquid velocity was m ore than twice the riser liquid velocity, depending on the gas velocity. This was on account o f the Ad/Ar ratio being 0.40. Since, the riser cross sectional area was m ore than tw ice the dow ncom er cross sectional area. For a given flow rate, a sm aller cross sectional area gives a higher velocity. W hile in the internal loop airlift (1) vessel, the dow ncom er liquid velocity was only slightly higher than the riser liquid velocity. This w as again m ainly due to the Ad/Ar ratio being 0.92. W hich m eant that the riser cross sectional area was nearly equal to the dow ncom er cross sectional area. The dow ncom er superficial liquid velocity w as roughly twice that o f the riser superficial liquid velocity in the internal loop airlift (2) contactor for a given gas velocity. The Ar/Ad ratio o f the internal loop airlift (2) reactor was 2.04.

0 .8 0 0 Internal (1) Internal (2) E xtern al 3 0 .4 0 0 0.200 Q . 0.000 0 .0 0 0 0 .0 0 5 0 .0 1 0 0 .0 1 5 0 .0 2 0

Superficial gas velocity (m/s)

0 .0 2 5

Figure 5.7: The variation in liquid circulation velocity in w ater w ith gas flow rate in the internal loop airlift (1), internal loop airlift (2) and the external loop airlift. The liquid is water. The data points w ith lines represent the riser liquid velocity, w hile the dow ncom er liquid velocity is given by the symbols.

0.200 Internal (1) Internal (2) % 0 .1 5 0 E xtern al "D 0 .0 5 0 0.000 0 .0 0 0 0 .0 0 5 0 .0 1 0 0 .0 1 5 0 .0 2 0

Superficial gas velocity (m/s)

0 .0 2 5

Figure 5.8: The variation in liquid circulation velocity in w ater with gas flow rate in the internal loop airlift (1), internal loop airlift (2) and the external loop airlift. The liquid is 82.0% glycerol solution. The data points with lines sym bolise the riser liquid velocity, w hile the dow ncom er liquid velocity is given by the symbols.

N um erous investigators (Bello e t a l . 1984; Chisti, 1989; Choi and Lee, 1993; Siegel

e t a l , 1988) have reported the strong influence o f the Ad/Ar ratio on the superficial liquid

velocity in airlift reactors. Choi and Lee (1993) show ed that the riser superficial liquid velocity increased nearly three fold w hen the Ad/Ar ratio o f their external loop airlift vessel w as changed from 0.11 to 0.53. In this study, for a given gas flow rate, the riser superficial liquid velocity in the internal loop airlift (1) was the highest (Figs 5.7 - 5.9). Generally, for a given gas flow rate, the riser liquid velocity in the internal loop airlift (1) was m ore than double the riser liquid velocity in the internal loop airlift (2). This was m ainly due to the larger riser cross sectional area in the internal loop airlift (2) com pared to the internal loop airlift (1). The riser superficial liquid velocity in the internal loop airlift (2) was slightly higher than in the external loop airlift for a specified gas flow rate. This was anticipated since the internal loop airlift (2) vessel had a slightly higher Ad/Ar ratio (0.49) to that o f the external loop airlift contactor (0.40). It m ust be rem em bered that both the internal loop

airlift contactors had a bottom section w hich was precisely designed to keep the frictional losses (at the bottom ) to a m inim um (Section 4.1.1). Chisti e t a l . (1988) claim ed that in

internal loop airlift reactors the frictional losses due to a change in the flow direction at the top is negligible. Com pared to the internal loop airlift contactors, in this work, the external loop airlift vessel had four 90° bends w hich w ould result in higher frictional losses than in the internal loop airlift devices.

The m ain driving force for the liquid circulation in airlift contactors was the density difference betw een the riser and dow ncom er sections. H ow ever, the m agnitude o f the superficial liquid velocities in the three airlifts is higher (Figs 5.7 - 5.9) than w ould be expected if it was ju st due to the density difference. Since in som e instances there was only a small density difference betw een the riser and dow ncom er. Ayazi Sham lou e t a l . (1994)

have explained that the liquid circulation velocity was also due to the entrainm ent and transport o f liquid in the w ake associated with the rising bubbles.

0 .3 0 0